The adjustment of the level of an audio signal is one of the most fundamental operations performed by audio signal processing equipment. In professional audio mixing equipment, for example, adjustments to signal level are made prior to an audio event and said adjustments are also implemented during the audio event itself, consequently, the process of mixing audio signals has become a specialised occupation in itself.
In a very basic mixing apparatus, a mixing desk operator may prepare a schedule of mixing operations required during the audio event from which he, possibly with help from assistants, follows during the event, making manual adjustments where necessary. Audio mixing involves the process of taking a plurality of audio input signals, processing each signal individually, which may involve filtering or other processing, to obtain the required tonality and mixing the signals together to create one or more outputs, such as a pair of stereo outputs.
The processing circuitry present for each audio input signal is commonly referred to as a channel and in recent years the number of channels provided by mixing consoles has increased significantly, it not being uncommon to find consoles with capacity for sixty four channels.
As far as an operator is concerned, adjustments to audio level are effected by the manual operation of a linearly moveable device, known in the art as a fader. In known audio mixing systems, the fader moves an electrical contact over a resistive track, thereby variably attenuating the audio signal. Furthermore, faders are known in which an additional track is provided from which data may be obtained defining the position of the fader during an audio event. This data may be recorded and then used to automatically control the position of the fader during an automated playback. Thus, faders arranged to operate in this manner are also provided with a motor for effecting automated movement of the fader and are generally referred to as motorised faders.
The availability of improved analogue integrated circuitry led to the development of mixing equipment in which the level of an audio signal could be controlled in response to stored data by means of a voltage controlled amplifier. Furthermore, the provision of such devices made the manual operation of the fader redundant, once gain control had been taken over by the aforesaid variable gain device, operating under the control of a computer. Thus, under computer control, audio level could be adjusted automatically in response to the stored data, irrespective of the position of the manually adjustable fader, which would remain stationery. The availability of the fader during automated playback led to the development of a system in which the fader could be used to make minor adjustments to the channel level during automated playback, thus allowing the operator to improve the quality of the stored data. This facility was provided on equipment manufactured by the present applicant and has been identified by the trade mark "trim mode".
Although providing additional facilities to the engineer, the introduction of automated control via variable gain devices also introduced disadvantages, when compared with the previously known approach of using the fader itself to manually adjust audio levels. Primarily, there has been a move to reduce signal degradation introduced by the audio signal path, which in turn has led to a movement away from active gain control devices, such as voltage control amplifiers and a return to the more traditional techniques using simple variable resistive elements. In addition, operators prefer to see the link between fader movement and audio level maintained, such that a variation in audio level is always associated with a movement of its respective fader. Thus, this movement may be achieved by initial manual operation of the fader or by automated control of the fader via its motorised elements.
Thus, to achieve automated audio level variations in a system using the resistive elements, it is necessary to automatically move the fader during automated playback. However, given that the fader is being driven by stored data and the audio signal itself continues to pass through the audio track associated with the fader, all gain variations must be effected by the fader and, consequently, the fader is no longer available to effect "trim mode". Such a restraint is commonly identified by referring to the loss in the fader, that is the attenuation applied to the audio signal passing therethrough, as being absolute.
An improved system is shown in Japanese patent JP-A-60061902 in which an automatically controlled motorised fader is driven in response to stored data. During the operation of the fader in this mode, further manual adjustment may be effected because, on touching the fader, the fader itself disconnects the motor and any further movements made manually by the fader result in new data being recorded. Thus, on the next cycle, the new data is used to control the automatic movement of the fader. However, in such a system, the relationship between fader movement and audio attenuation still remains absolute and the system is not as versatile as the previously used systems with variable gain control.